1. Technical Field of the Invention
The present invention relates to a spark plug for an internal combustion engine, which is used such as for an automotive engine.
2. Related Art
A spark plug is used as an igniting means in an internal combustion engine, such as an automotive engine. For example, in such a spark plug, a center electrode and a ground electrode are arranged opposed to each other in the axial direction of the spark plug to form a spark discharge gap therebetween. In such a spark plug, discharge is permitted to occur in the spark discharge gap so that the discharge can ignite air-fuel mixture in the combustion chamber.
In the combustion chamber, a flow of air-fuel mixture, such as a swirl flow or a tumble flow is formed. The flow is also moderately led to the spark discharge gap to ensure ignitability.
A spark plug is mounted in a posture to an internal combustion engine. Depending on the posture of the spark plug with respect to the internal combustion engine (hereinafter referred to as a “mounted posture”), a part of the ground electrode connected to an end portion of a housing may be located upstream of the spark discharge gap within the flow of air-fuel mixture. With this location, the flow of air-fuel mixture in the combustion chamber is blocked by the ground electrode and accordingly, the flow in the vicinity of the spark discharge gap is likely to stagnate.
As a result, ignitability of the spark plug may be deteriorated. Specifically, depending on the mounted posture of the spark plug with respect to the internal combustion engine, ignitability of the spark plug may be problematically varied. In recent years, in particular, there is a growing trend of using a lean-burn internal combustion engine. In such an internal combustion engine, combustion stability is likely to be deteriorated depending on the mounted posture of the spark plug.
In addition, it is difficult to control the mounted posture of the spark plug with respect to the internal combustion engine, i.e. to control the location of the ground electrode in the circumferential direction of the spark plug (i.e. mounting angle). The mounted posture is varied depending such as on the mounting state (for example, mounting angle) of the screws used for mounting the spark plug in a housing, or the degree of tightening the screws in the work of mounting the spark plug to the internal combustion engine.
A patent document JP-A-H05-315049 discloses a spark plug in which a spark discharge gap is formed being distanced from the body of the spark plug as much as possible, so that the spark discharge gap is exposed to a larger amount of air-fuel mixture and that fresh air-fuel mixture can easily flow into the spark discharge gap.
However, it is also difficult for the spark plug disclosed in JP-A-H05-315049 to avoid the problem set forth above. That is, the problem that ignitability of the spark plug is varied depending on the mounted posture of the spark plug with respect to the internal combustion engine, i.e. the location of the ground electrode in the circumferential direction of the spark plug. In the configuration disclosed in JP-A-H05-315049 as well, the ground electrode or the center electrode is located upstream of the spark discharge gap, depending on the mounted posture of the spark plug, causing the flow of air-fuel mixture to stagnate in the spark discharge gap. In this way, ignitability of the spark plug is likely to be varied depending on the mounted posture of the spark plug.